The present invention relates to a combination of a balloon and a stent with specific sizes, wherein the stent is opened substantially to the maximum opening size by the balloon with substantially the same maximum opening size to provide an effective retaining or supporting ability to the stent in a vessel.
The most of the current stents on the market including coronary and peripheral stents are designed to have struts, and elbows or bending portions, wherein the struts in one strut section are bent at the elbows and arranged to have a zigzag shape as a whole. Some of the elbows in one strut section are connected to the elbows in the adjacent strut section. When the stent is in a closed condition, the struts are situated substantially parallel to the stent axis. As the stent begins to open by inflating a balloon on which the stent is mounted, an angle between two struts connected with each other through the elbow in between becomes wider.
In general, all the currently available stents are designed to cover all sizes by one or two stents in a coronary market. The range of the diameter when it is opened is between 2.5 mm and 4.0 mm.
There are two factors in determining the size of the stent in the open condition. One is the size of the balloon to be used, and the other is an inflating pressure applied to the balloon. For example, if a balloon with 3.0 mm in diameter is used, the stent will open to 3.0 mm in diameter. If a higher pressure is applied, the balloon may expand slightly more, so that the stent may become 3.2 mm in diameter.
The stents are generally sold in a package, so-called pre-mount stents. In the pre-mount stents, the stents are already mounted on various balloons, which respectively have various diameters, such as 2.5 mm, 3.0 mm, 3.5 mm, and 4.0 mm. Although the balloons are different in the pre-mount stents to equally open the stents in the various diameters, a common stent which opens to various diameters is mostly used for those balloons having different diameters.
More specifically, as shown in FIG. 4, a conventional stent 101 used for all types of the balloons has a cylindrical shape and a diameter D101 in a closed condition, and mounted on a balloon 102 of a balloon catheter 103, known in the art, as shown in FIG. 5.
After the stent is delivered to a specific position by the balloon catheter 103, an inflating pressure is applied inside the balloon 102 so that the balloon 102 is inflated to have an opening diameter D102 as shown in FIG. 6. Accordingly, the stent 101 mounted on the balloon 102 is opened by the balloon 102 to have the opening diameter D102. For example, the diameter D102 is 2.5 mm. In the open stent shown in FIG. 6, an angle formed by a strut 104 and an axis A parallel to a central axis of the stent 101 is 40 degrees.
When the stent 101 is mounted on a balloon 102' of a balloon catheter 103' in which an opening diameter of the balloon 102' is larger than D102, the stent 101 is further opened to have a diameter D103 which is larger than the diameter D102. For example, the diameter D103 is 3 mm, and an angle formed by the strut 104 and the axis A of the stent 101 is 50 degrees as shown in FIG. 7.
As described above, since the stent 101 having the same size, such as the diameter D101, in the closed condition is used for the various balloons 102, 102' having various diameters D102, D103 in the opened condition, the inflating pressures applied inside the balloons 102, 102' are increased and the angles between the strut 104 and the axis of the stent 101 becomes larger as the diameter of the balloon becomes larger.
Namely, if the stent is opened to have a diameter close to the maximum opening diameter, the strut angles, as mentioned above, become obtuse. Thus, it requires more pressure to open the stent to have the maximum diameter. This is very desirable to the stent since it takes more pressure to close or collapse the stent. In other words, when the stent is opened to the maximum diameter, the stent has a high withstanding force against collapsing.
Thus, in the examples shown in FIGS. 6 and 7, the stent 101 shown in FIG. 7 has the withstanding force against collapsing stronger than that shown in FIG. 6. In this sense, although the same stent 101 is used, the ability for supporting is different according to the diameters of the balloons used in the blood vessel.
On the other hand, there is a prior art publication indicating that the struts are bent perpendicular to the stent axis, which is most desirable in supporting the blood vessel. Although this statement is true, it is almost impossible to expand the struts perpendicular to the axis of the stent by the balloon because it requires too high inflating pressure by the balloon. The relationship of the angle of the strut relative to the pressure applied to the strut is show below, wherein the angle is measured as in FIGS. 6 and 7.
Angle(degrees) Pressure(ATM) 15 1.00 30 2.15 45 3.73 60 6.46 75 13.93 80 21.17 85 42.66
As shown above, it requires about 42 ATM to open the strut at 85 degrees. This is practically impossible to open the strut by the balloon. The balloon now available in the market can provide 12 to 20 ATM.
The present invention has been made in view of the above, and an object of the invention is to provide a combination of a specific size balloon and a specific size stent, wherein each stent is designed to open to the specific one size only to prevent collapsing of the stent after enlargement.
Another object of the invention is to provide a combination of the balloon and stent as stated above, wherein the stent is enlarged to the maximum size as designed to be used for a long time without trouble.
Further objects and advantages of the invention will be apparent from the following description of the invention.